1. Field of the Invention
The present invention relates generally to surface mount technology (SMT) as used in manufacture of electronic products, and is particularly concerned with electronic package assemblies comprising surface mount components and/or subassemblies (SMT devices), and with integral heater assemblies and methods for heating solder layers securing SMT devices to allow removal from host boards.
2. Related Art
The trend in electronics packaging is to eliminate discrete or separate electronic components and make assemblies smaller by relying on very large scale integration (VLSI) integrated circuits or electronic modules. This allows designers of printed circuit assemblies and products to purchase a module for a particular desired function from a manufacturer, without having to design a circuit to perform the function and in a very compact form. In a module, several chips or other components are usually present in a miniaturized form. For example, a module might include a complete intermediate frequency (IF) strip, a voltage controlled oscillator (VCO), a global positioning system (GPS) receiver, a cell phone transceiver, or some other complete assembly.
Modules are relatively expensive, high value devices, and are frequently printed circuit board or PCB based, in other words the assembly is based on a carrier board, which serves as a substrate to hold the internal components and also provides a metallized mounting plane or solder interface layer, and can be thought of as a “module” which is in turn soldered to a host board which is larger than the module and may host many other components and modules. Traditionally, all the components are mounted to a host board using surface mount technology (SMT) construction techniques. Designers of host boards leave little space around the modules in an attempt to miniaturize the product, or leave space for other devices on the board. Currently, such products are getting smaller and space constraints are increasing.
SMT modules can be particularly difficult to remove from the host board. Removal might be required to facilitate troubleshooting, or to replace defective modules. Sometimes, defective modules are returned to the manufacturer for warranty service or credit. During installation of SMT modules and other components onto a host board, the entire host board is heated to reflow solder paste on top of the host board and under the SMT modules. In order to remove an individual module from the board after installation, it is desirable to heat only a selected portion of the host board, but that is rarely possible. All of the solder under a given SMT module must re-flow simultaneously in order to remove it. Heating the entire host-board assembly past the solder reflow temperature jeopardizes the quality of the unit by wasting valuable component termination leach life, degrading solder quality. In some cases, heating the entire board is not even possible because a secondary re-flow to facilitate replacement of a single component or module will cause other components to become damaged or to fall off. Some companies offer local heat systems, usually based on hot air, such as the Hakko 850 SMT Rework Station hot air nozzle, manufactured by American Hakko Products, Inc. of Valencia, Calif. Such systems do not work well with large modules or modules with poor thermal conductivity from top to mounting plane. The hot air temperatures necessary to reflow the solder which is under the module usually damages the components inside. Typical temperatures required for solder reflow are generally of the order of 183° C. to 240° C. (higher temperatures being associated with lead-free solders), and in order to reach this temperature under a module, the heat source must be considerably hotter, particularly in view of heat drains in the module causing convective and conductive heat losses. The hot air stream used for module removal can be as hot as 700° C.
Many SMT modules are very conservatively rated with respect to temperature exposure. For example, standard (non lead-free) ceramic microwave filter modules from Integrated Microwave Corp. (IMC) of San Diego, Calif. are rated to 215° C., which prevents filter module removal without damage to the filter itself. It is rare that a filter module that has already been installed can be removed undamaged. Hot air at 215° C. cannot overcome the thermal masses and losses associated with a typical mounting configuration and still reflow solder under the module, which probably melts at 183 C or higher, and therefore the module cannot be removed without damage. Even preheating the host board to assist in overcoming these losses, which is the most successful tactic, rarely produces the desired result and usually results in damage to one or more modules which were previously surface mounted on the board.
Reinstallation of a module is also rarely possible using current state of the art techniques. Frequently, a manufacturer will cheat and use a soldering iron to re-install a module. The result is a solder joint around only the periphery of the module, and no solder reflow underneath. The resulting air gaps under the module degrade thermal performance, degrade RF performance, provide inferior mechanical strength, and are generally known to be undesirable. In RF filter module installations above approximately 300 MHz, this practice is particularly detrimental and degrades return loss (VSWR) and rejection (attenuation of undesired RF energy) as a result. Some solder process engineers ban the practice of using a soldering iron for module re-installation for these reasons.
Another common option for module removal is use of a so-called “hot shoe”. Basically, a hot branding iron or shoe is placed on the board next to the device to be removed, which may be a module or individual component. The shape of the iron is selected to fit in the allowable space. It is part of an effort to heat the solder under the device, rather than the device itself. The shoe makes contact with the peripheral space immediately surrounding the module, and requires exposed solder for contact. There are many limitations to hot shoe technology. For example, the shoe must have the right shape, and there has to be space for it to fit around the device, and a space for the device to occupy without contacting the shoe. This means that custom parts need custom shoes. Reinstallation of modules or components using this technique is very difficult or impossible. Host board damage is likely. The shoe must be very hot, like a soldering iron (600-700° F.). For large mass modules, the time needed for module removal makes host board damage inevitable. For large area modules, the technique does not work at all because reflow cannot be achieved near the center of the module, and therefore it cannot be removed.
Some attempts have been made to overcome the problems in removing or reworking multi-chip modules (MCMs) from a larger printed circuit board using known removal techniques, such as heat guns. An MCM typically has a multi-layer substrate of high temperature, co-fired ceramic. U.S. Pat. No. 6,031,729 of Berkely describes providing multiple side-by-side electrical heaters on the lower layer of a multi-chip module (MCM) having a multi-layer substrate of high temperature, co-fired ceramic. The heaters are formed by a plurality of resistive tungsten metal traces. Tungsten metal is selected in this case because it is compatible with the high firing temperatures inherent in ceramic substrate fabrication, and Berkely states that this material can be tailored to produce the required resistance for heater elements. The technique described in this patent is limited to MCMs with ceramic substrates and printed wiring of tungsten or like materials having lossy characteristics akin to that of tungsten. Tungsten is a high resistance, exotic material and is both expensive and difficult to process. Traditional epoxy-glass circuit board materials (such as FR-4) are not available clad with tungsten, and the addition of tungsten would make production and processing virtually impossible.
The known heating methods involving multiple heater elements are not applicable to many types of electronic components including larger scale surface mount modules.